This invention relates to a magnetic length or angle measuring instrument for measuring the relative position of a first object with respect to a second object, of the type comprising means for defining a measuring division of alternating magnetically conductive and magnetically non-conductive zones, a scanning unit mounted to scan the division, and means for evaluating signals generated by the scanning unit to determine to the relative position of the scanning unit with respect to the measuring division.
A variety of such magnetic position measuring instruments are known to the art. In West German Pat. No. 24 28 785 there is disclosed a magnetic measuring instrument for determining the position of a rod which is movable along a longitudinal direction. This cylindrical rod is formed of a magnetically conducting material and is provided on its circumference with grooves which are formed in a winding-line pattern. These grooves are filled with a magnetically non-conducting material. The rod and the material in the grooves form a division of magnetically conducting and non-conducting sections extending lengthwise along the rod. This division is scanned by a scanning unit which is arranged radially, symmetrically around the rod. This scanning unit includes a permanent magnet, pole shoes for scanning the division, and magnetic field sensitive sensor elements. In this system, the scanning unit includes several scanning regions that are distributed around the circumference of the rod in a plane perpendicular to the axis of the rod. This measuring instrument provides the disadvantage that, in the case of relatively large measuring lengths, intermediate supports for the rod are not possible, since the scanning unit surrounds the entire circumference of the rod. This instrument arranges the permanent magnet with its magnetization direction extending along the longitudinal extent of the rod. By reason of the finite hysteresis of the rod material, relatively large reversal intervals occur in the electric signals generated by the magnetic field dependent sensor elements, thereby impairing measuring accuracy. Furthermore, the scanning unit is sensitive to external magnetic interference fields because of the spatial arrangement of the scanning regions.
The publication "The Numeral Control of Machine Tools", Simon, 2nd Edition, 1971 describes in Section 4.4 a magnetic measuring device called the Accupin. This device includes a scale made up of a plurality of thin cylindrical pins of magnetically conducting material which are mounted in the grooves of a metallic bar formed of a non-magnetic material. The scanning unit includes a plurality of coils, the inductivity of which is modulated by the scale. Since the scale described in this publication includes a large number of individual parts, it is relatively expensive to manufacture and may not be sufficiently sturdy with respect to mechanical considerations for some applications.